[InstCombine] fix matching of or-of-icmps constants (PR32524)

Also, make the same change in and-of-icmps and remove a hack for detecting that case.

Finally, add some FIXME comments because the code duplication here is awful.

This should fix the remaining IR problem noted in:
https://bugs.llvm.org/show_bug.cgi?id=32524

llvm-svn: 299851